Coatings for high-temperature alloys



J ly 11, 1967 A. D. JOSEPH ETAL 3,330,633

COATINGS FOR HIGH-TEMPERATURE ALLOYS Fild June 26, 1964 4 Sheets-Sheet 1 WEIGHT CHANGE VERSUS TIME FOR DIFFERENT COATINGS 0N Ni BASE SUPERALLOY. ALL SAMPLES 0N SMZOO ALLOY I00 HOURS AT 2000F WEIGHT 004 comma OEXAMPLE 1 Mo Sig Mi Al v INVENTORS 9 9 2 A. DAVID JOSEPH o Nl-ZOCI (BY WEIGHT) BY FRANKR TALBO0M,JR.

,QW M, M "LC/Q A TTOR NE Y5 y 11, 1967 A. D. JOSEPH ETAL 3,330,633

COATINGS FOR HIGH-TEMPERATURE ALLOYS Filed June 26, 1964 4 SheetsSheet WEIGHT CHANGE VERSUS TIME FOR DIFFERENT commas H62 0N Ni BASE SUPERALLOY. ALL SAMPLES ON SM 200 ALLOY-I00 HOURS AT 2|0OF WEIGHT 7 GAIN IN GRAMS 80 ICO TlME (HOURS) WEIGHT IN GRAMS comma ZXASMP'LE 1 INVENTORS B A. DAVID JOSEPH C BY FRANK P. TALBO0M,JR. O 0 r2 I 0 Ni- 20 Cr BY WEIGHT) LW,M, fi

ATTORNEY! y. 11, 1967 A. D. JOSEPH ETAL 3,330,633

COATINGS FOR HIGH-TEMPERATURE ALLOYS 4 Sheets-Sheet 5 Filed June 26, 1964 WES 2528 5552525 NEON 2:28 5.50

MZQN 8:28 E252 2 5 mass m at y 11, 1967 A. D. JOSEPH ETAL 3,330,633

COATINGS FOR HIGH-TEMPERATURE ALLOYS Filed June 26, 1964 4 Sheets-Sheet 4 FIGS EROSION TEST BARS AFTER OXIDATION- EROSION TEST OF IOO HOURS AT 2000F UNCOATED I Al-IO 31 SM 200 IBY WEIGHTI EXAMPLE I MoSi NiAI: Ni-2O Cr TuCr (BY WEIGHT) EROSION TEST BARS AFTEROXIDATION- EROSION TEST OF IOO HOURS AT 2000F AND IOO HOURS AT 2IOOF for engine parts become much more United States Patent COATINGS FOR This invention relates to novel coatings for nickel-base alloys having nickel as their principal component that will protect such alloys from oxidation at high temperatures and to a method for creating such coatings.

More particularly, this invention relates to a tantalummodified nickel-aluminum composition coating for nickelbase alloys having nickel as their principal component. The coating is created by depositing an intimate mixture of tantalum and aluminum powders on the surface of a substrate by dipping, painting or spraying it on in the form of a slurry or dispersion in an organic solvent. The powder coveredsubstrate is then sintered in a reducing, inert or vacuum atmosphere furnace to cause fusion of the powder mixture onto the substrate. The invention also particularly relates to a two-step process for creating an improved form of such coatings in which the tantalummodified nickel-aluminum compositions forming the coating achieve a Stratified or layered structure and improved ductility.

The coatings of this invention provide excellent long term protection to nickel-base substrates at metal temperatures up to 2000 E, medium term protection at metal temperatures up to 2100" F., and short term protection at metal temperatures of 2200 F., or more under conditions of high velocity gas erosion such as typically are encountered in a gas turbine engine.

It is noteworthy that although the coatings of this invention provide highly superior protection for nickel-base alloys, they are not generally applicable to alloys having cobalt as their principal component.

Although nickel-base alloys typical of those in current use do not begin to melt until a temperature of about 2380 F, is reached, such alloys in gas turbines, if unprotected, fail rapidly at turbine inlet temperatures of 1800 F. or above. The mechanism of failure is by preferential intergranular oxidation attack at grain boundaries or between grain boundaries. Penetration at grain boundaries leads to notches at the loci of penetration, and stresses created at these notches in turn can lead eventually to mechanical failure of the part. An important function of the coatings of this invention is to prevent such intergranular oxidation attack on nickel-base alloys.

Virtually since the introduction of jet aircraft engines during World War II, pressures have existed for their constant uprating. The impetus for uprating has been in large part created by the fact that slight increases in turbine inlet temperatures can provide significant increases in thrust. In turn, slight increases in thrust yield important increases in engine efficiency and economy, but as turbine inlet temperatures are increased performance requirements demanding.

Current engines using parts with available coatings are rated at turbine inlet temperatures of about 1900 F. More advanced engines being made today operate at turbine inlet temperatures of about 2000 F. for constant operation, and work is being done on engines designed to .run at turbine inlet temperatures of about 2100 F. Metal temperatures of turbine blades are nominally 250 to 300 F. below turbine inlet temperatures in a given engine, but hot spots can be caused in blades and they may gothrough heat zones that cause them to reach turbine inlet temperatures. Momentary engine overshoots, or sudden but brief increases in turbine inlet temperatures caused, for example, by large thrust demand on takeoff or by a spurt of fuel admitted to the combustion chamber at any time during operation, can result in increases in turbine inlet temperatures of as much as 300 F. above constant operation temperatures. Such overshoots can cause corresponding temporary increases in turbine blade metal temperatures of about 300 F. above normal. A clear need thus exists for higher temperature coatings that will give good protection to nickel-base alloys at metal temperatures up to at least about 2100 F.

The highest turbine inlet temperature that uncoated blades can stand without rapid failure due to high velocity gas erosion is about 1800 F. The better existing coatings, such as Al-lOSi, will protect blades at turbine inlet temperatures up to about 1900 F. By contrast, however, the coatings of this invention give superior protection to nickel-base alloy blades and vanes against high velocity gas erosion for times up to 5000 hours or more at turbine inlet temperatures of at least 2100 F. The coatings of this invention will also give short time protection to nickel-base alloys at turbine inlet temperatures up to at least 2400 F., thus affording reliable protection against momentary engine overshoots.

Expressed in terms of metal temperatures, the coatings of this invention provide protection for nickel-base alloys at temperatures up to about 2200 F. for times of about 50 hours with ability to protect at even higher temperatures for shorter periods of time. They achieve a coating life of over 300 hours at metal temperatures of about 2000 F. and a life of many thousands of hours at metal temperatures of about 1800 F.

As engine temperatures go up, problems multiply. This invention meets the need fora superior coating that will fulfill the requirements imposed by higher engine operating temperatures. The final product of this invention achieves both an unexpectedly high surface melting point and outstanding oxidation resistance. In protecting turbine blades at higher operating temperatures, melting temperatures of coatings are of overriding importance. Basically, coatings must of course be oxidation resistant, but once oxidation resistance is achieved, such as with current aluminide type coatings-on nickel-base substrates, the relatively low melting points of such prior coatings becomes a severely limiting factor preventing further increase in turbine inlet temperatures. There has thus been a long felt need for coatings having higher melting points that would be able to withstand the exigencies of higher engine'oper ating temperatures without failure.

Existing coatings furnish adequate oxidation resistance at turbine inlet temperatures up to 1900 F.,. but when the turbine inlet temperature is moved up to 2100 F., these coatings become subject to melting by exposure to hot spots or momentary engine overshoots, Characteristically, coatings on nickel-base substrates tend to soften at a temperature below their melting point. The closer the melting point is approached, the softer the coating becomes. As exposure temperatures of coatings are increased, erosion is accelerated by softening of the coating. Coatings thus can be caused to fail by gross erosion when exposed to high velocity turbine gases at temperatures appreciably below their melting points, a characteristic that again emphasizes the importance of a high melting point for a satisfactory coatin A concomitant problem has been to achieve a coating that in spite of having a high melting point can nevertheless be applied at a temperature that is compatible with the heat treating temperature of nickel-base substrates. In most nickel-base alloys for turbine blades a good temperature for initiation of heat treating is about 1975 F Ideally, than a coating for such blades should be capable of being applied at this temperature. It is an unexpected beneficial result of this invention that the coatings taught can be applied at the relatively low heat treating temperatures characteristic of nickel-base substrates but still yield coatings having much higher melting points than their application temperatures.

A further benefit provided by coatings that can be applied at the heat treating temperatures of their substrates is that the coating and heat treating processes can be accomplished simultaneously. This attribute promotes economy and efficiency in production of key engine parts.

As engine operating temperatures are increased, oxidation resistance is lowered; erosion is increased; sulfidation attack is caused by sulfur compounds in the combustion gases and fuels; and more frequent inspection and replacement of engine parts is required. Although they extend the life of engine hardware, current production coatings do not provide the protection and longitivity required for extended use of engines in the 1800-2200 F. turbine inlet temperature range. Such coatings are inadequate for these higher engine operating temperatures, because as turbine inlet temperature is raised these coatings display the following inadequacies:

(1) Excessive interdiffusion between coating and substrate takes place with consequent dilution of coating composition and lowering of its protection potential.

(2) Melting points of existing coatings are too close to metal temperatures experienced in higher temperature engines.

(3) At such temperatures existing coatings offer insufficient oxidation resistance.

(4) When their melting points are closely approached, such coatings suffer from excessive gas erosion by the turbine gas stream.

(5) Some existing coatings become highly susceptible to spalling as engine temperatures are increased.

The useful upper limits of current production coatings, such as, for example, Al-lOSi (aluminum plus weight percent silicon), are restricted by melting temperatures of about 2050 F. When attempts are made to raise turbine inlet temperatures above 1900 F., these coatings fail to provide adequate oxidation resistance. The closer the melting point of a coating is approached, the more rapidly interdififusion between coating and substrate takes place. As temperatures are increased the aluminum, for example, in prior coatings tends to diffuse into a nickelbase substrate, and this undesirable diffusion of aluminum reduces its availability at the coating surface. The lack of aluminum at the surface prevents formation of nickelaluminum spinel (NiAl O as a pellicular film on the outer surface of coated substrates, and it is the nickelaluminum spinel that is believed to provide primary oxidation resistance.

In view of the foregoing, it is a primary object of this invention to provide as a new and improved article of manufacture a nickel-base alloy substrate having a tantalum-modified nickel-aluminum coating and a process for achieving such an article in which the coating has a higher melting point than any known aluminum type coating for nickel-base alloys. Such tantalum-modified aluminide coatings on nickel-base substrates exhibit an unexpected beneficial increase in melting temperature and a corresponding increase in diffusional stability and resistance to interdilfusion between coating and substrate. The unexpected increase in melting point of the coating brought about by the tantalum-modification is singularly important in achieving this new and superior coating.

Another object of this invention is to provide for nickelbase alloys, a new and improved tantalum-modified nickel-aluminum coating composition that has a melting point well in excess of 2380 F., or about the upper limit of temperature to which existing nickel-base alloys can be exposed without melting or unacceptable softening of the substrate. The coatings of this invention thus provide a nickel-aluminum coating composition that boosts the effective temperature of aluminum type coatings well beyond their prior capabilities which were severely limited by their inability to exceed a melting temperature of about 2050 F.

It is another object of this invention to provide a superior coating for nickel-base alloys that can be applied by sintering at heat treating temperatures for such alloys but when once applied will have a melting point well above such temperatures.

A further object of this invention is to provide a new and improved coating composition for nickel-base alloys that has a high melting point and also possesses room temperature ductility. The latter characteristic of such coatings make them capable of deforming with indentations or defects imposed on the coated parts thus making the coatings resistant to failure from ballistic impact at low temperatures.

Another object of this invention is to provide a new and improved coating for nickel-base turbine vanes and blades that will enable them to be operated at temperatures where they can perform more efficiently and still be protected from failure through intergranular oxidation attack.

Yet another object of this invention is to provide superior coatings for nickel-base alloys that can be applied as an intimate mechanical mixture of finely divided powders dispersed in a suitable liquid dispersant or slurry which can be put on the part to be coated by dipping, painting or spraying. Parts to be coated can thus be masked to achieve selective coating or specific areas. This is an important attribute in coating turbine blades, since tight tolerances are normally preserved between the blade root and disc on which the blade is mounted. By selective masking of blade roots during the coating process the coating can be prevented from interfering with desired production tolerances on blade roots.

Additional objects and advantages will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention, the objects and advantages being realized and attained by means of the compositions, methods and processes particularly pointed out in the appended claims.

As set forth in the description of the invention and the claims, it will be understood that the term diffusing temperature refers to temperatures of from 1800 to 2250 F. Although not well understood, it is known that after a period of exposure at such temperatures the tantalum-modified as-sintered coatings of this invention change their structure from essentially a relatively fine grained dispersion of modified NiAl in a matrix of a nickel-aluminum composition to a stratified or layered structure in which the outer metal coating zone is an aluminum rich nickel-aluminum composition similar to Ni Al the intermediate metal coating zone is a nickelaluminum composition similar to NiAl, and the inner metal coating zone is a nickel rich nickel-aluminum composition similar to Ni Al, all modified by tantalum and in which the inner metal coating zone is present as a reasonably clearly defined stratum between the two strata forming the outer and inner zones.

The speed with which this structural change takes place is strongly influenced by time temperature relationships. For example, virtually complete Stratification may take only 2.50 hours to achieve at 2250 F. but it may require 25,000 hours of exposure to achieve it at 1800 F. When the coatings of this invention assume a partially stratified structure after exposure to a diffusing temperature for times of about 250 hours or more, they possess a preferred structure and manifest increased ductility.

The term oxidizing temperature refers to those temperatures at which formation of a pellicular outer film of nickel-aluminum spinel takes place on the surface of the as-sintered tantalum-modified nickel-aluminum coating composition of this invention in an oxidizing environment. Such oxidizing temperatures are from 1400 to 2300" F. Exposure to oxidizing temperatures is normally effected dun'ng actual use of the coated articles and not as a separate step.

Exposure to a diffusing temperature may be effected as a separate step in either an oxidizing or nonoxidizing environment, may be simultaneous with actual use of coated parts in a gas turbine, or may be achieved by a combination of these methods. The longer coatings are exposed at temperatures of 1800 F. or greater, the more coating ductility increases, i.e., coating ductility of the coating increases directly with length of exposure at temperatures of 1800 F. or above.

To achieve the foregoing objects and in accordance with its purpose, this invention includes an article of manufacture having good stress-rupture strength at high temperatures, high-temperature oxidation resistance, and resistance to cyclic fatigue failure which comprises a substrate consisting essentially of a nickel-base alloy having nickel as its principal component, the article having a defect, oxidation, interdiifusion, thermal shock, melting, and erosion resistant surface zone consisting essentially of a nickel-aluminum composition having an atomic ratio of nickel to aluminum of from 2:3 to 3:1 and modified by from 1 to 10% tantalum by weight of total composition, the surface zone being further characterized by ductility at room temperature, and a melting point higher than that of the substrate.

As used in the specification and claims the term nickel base alloy will be understood to include those alloys in which nickel is the principal component and is present in an amount of not less than 40% by weight of the alloy.

The invention further comprehends a method of producing a coated metal article having good stress-rupture strength at high temperatures, high-temperature oxidation resistance, and resistance to cyclic fatigue failure, the article comprising a metal substrate consisting essentially of a nickel-base alloy having nickel as its principal component, and the method comprising the steps of: contacting the substrate with a mechanical mixture of finely divided powders consisting essentially of tantalum in an amount of from 50 to 80% by weight and the balance aluminum, placing the substrate while in contact with the metal powders in an inert or vacuum atmosphere, heating the substrate while in contact with the metal powders to a sintering temperature of from 1800 to 2100 F. for a time period sufficient to create a tantalum-modified nickelaluminum composition coating on the substrate.

Such process may be supplemented by thereafter heating the coated article to a temperature of from 2100 to 2250 F. for about 250 hours or more to create an essentially stratified structure in the coating including three strata of tantalum-modified nickel-aluminum compositions.

The tantalum and aluminum powders used to produce the coatings of this invention usually have a size range of les than 325 mesh (43 microns) although coarser particles, ranging in size from about 100 mesh 147 microns) to 325 mesh may also be used. Especially good results are obtained when the size range of the tantalum and aluminum powders is less than 400 mesh (38 microns), or between about to 38 microns, and preferably between about 0 to microns. In general, it can be said that the finer the particles, the better the coatings produced. The mesh sizes reported are Tyler standard.

The metallic dusts or powders or tantalum and aluminum described above may be applied to a nickel-base alloy part, metal core or substrate to be treated in any suitable manner. A fine film of the tantalum and aluminum powders may thus be blasted or dusted on to the specimen; or a dispersion of the powders in a solvent liquid may be applied to the substrate after which the solvent may be evaporated leaving a coating of the powder mixture on the substrate. Other methods of applying the tantalum and aluminum powder mixture will readily suggest themselves to persons skilled in the art.

Before coating, the surfaces of the nickel-base substrate should be thoroughly cleaned of dust or dirt, as by water rinsing, liquid blasting, washing in suitable organic and inorganic solvents, and any other method of cleaning that is standard in the art. Care should be taken in cleaning the substrate to ensure that it is not injured.

In accordance with a preferred embodiment of this invention, a tantalum and aluminum powder mixture is dispersed in a suitable liquid dispersant, and the resulting dispersion is applied to the substrate by spraying, brushing, dip-coating, or any other conventional method.

The ratio of tantalum and aluminum powder mixture to liquid dispersion may vary from about 25 to 50% by weight, or higher. The liquid dispersant may be any suitable, readily volatilizable organic solvent, or mixture of solvents. Among the solvents that may be used are alcohols, such as methyl, ethyl, propyl, and butyl alcohol, esters such as methyl, ethyl, propyl, butyl and amyl acetate, and ketones, such as acetone.

The organic solvents mentioned are illustrative and not limiting. It should be understood that almost any volatile liquid that will act as a suitable dispersant for the tantalum and aluminum powder mixture can be utilized, and use of any such liquid is contemplated. The main requirement of the volatile liquid substance or dispersant is that it be reasonably safe to use, inexpensive, and sufiiciently liquid at ordinary temperature to act as a dispersant for the metallic powders so that the dispersant can be sprayed or suitably coated on the specimen, and at the same time be sufiiciently volatile to evaporate when exposed to atmospheric or other conditions as will be described below.

If described, a binder or sticking agent may be added to the liquid dispersant to hold the powder mixture to the surface of the substrate after evaporation of the solvent. Use of a binder enables the powders to adhere to the substrate for prolonged periods of time, thereby precluding the necessity of heat treating immediately after application or of taking special precautions in handling the treated substrate. The binder should be one that will be substantially completely decomposed during sintering or at a temperature below actual sintering temperature. Suitable binding or sticking agents that may be mentioned include nitrocellulose, naphthalene, and stearates. Other sticking or binding agents will be readily apparent to those skilled in the art.

Suitable wetting agents may also be added to the dis persant if required.

The dispersion of tantalum and aluminum powders described above in either a liquid or laquer dispersant, i.e., a dispersant containing a binder or sticking agent, is deposited on the surface of the specimen to be coated in the manner already described. After application, the solvent is allowed to evaporate thereby leaving a layer of tantalum and aluminum powder mixture on the substrate.

If a sticking agent is added to the dispersant, upon evaporation of the solvent, the sticking agent will remain dispersed throughout the dust or powder in the coating, and will serve to hold the powder or dust to the substrate.

Evaporation of the volatile solvent, or volatile portion of the lacquer, containing a sticking agent, may be conveniently brought about by allowing the coated substrate to be stored in an atmospheric environment at ordinary room temperature. If desired, suction or vacuum and elevated temperatures, may be used to accelerate evaporation of the volatile solvent. Evaporation of the solvent leaves a fine layer of tantalum and aluminum powder mixture on the surface of the substrate including any walls or sides defining interstices, slots, holes, and so forth thatmay be present in the substrate.

When a hinder or sticking agent is added to the liquid dispersant, the coating layer, upon evaporation of the Solvent, comprises a uniform intermixture of coating powder interspersed throughout the nonvolatile binder or sticking agent. The dried coating adhering to the specimen comprises metallic particles and binder, the metallic powder being suspended in or interspersed throughout the binder.

Preferably the mixture of tantalum and aluminum powders is formed into a slurry with the dispersant and hinder or sticking agent. The substrate can then be dipped into the slurry or the slurry can be sprayed or brushed onto the substrate. The substrate in turn may be masked in selective areas to prevent adherence of the slurry or dispersion to such masked areas and prevent the formation of any coating on such areas during sintering.

The thicknesses of the applied powders may vary from substrate to substrate, but in general, a sprayed thickness of from 3 to mils is contemplated. Such sprayed or brushed on coatings lead to a coating thickness after sintering of from about 1 to 3 mils. Preferably the thickness of the resulting coatings of this invention after sintering is about 2 mils.

When evaporation of the solvent has been completed, the resulting specimens are heat treated in a suitable furnace or oven to permanently fix the powder mixture to the specimen. The temperature of the furnace or oven is maintained at its sintering temperature. By sintering temperature is meant a temperature that will permanently bond the powder mixture in the sprayed-on application to the substrate by fusion.

For the coatings of this inevntion, sintering temperatures of from 1800" to 2100 F. are contemplated and a sintering temperature of 1950 to 2000 F. is preferred, because at such temperature the nickel-base alloy of the substrate may simultaneously be effectively heat treated while sintering is taking place.

The sintering period may vary from about 1 hour to 20 hours or more. Especially good results are achieved however, when the sintering is carried out for about 4 hours, and this period may be considered as optimum.

In accordance with the present invention, superior coatings are produced by sintering in a hydrogen atmosphere furnace. The sintering is carried out under atmospheric pressure, or preferably at a pressure slightly greater than atmospheric. The hydrogen atmosphere is particularly critical and should have a maximum dew point of -40 F. or less, preferably 60 F. It is important that the hydrogen be as completely free of oxygen as possible.

Of the essence of the present invention is a dramatic and unexpected increase in the effectiveness of nickelaluminum composition coatings on nickel-base alloy substrates achieved by inclusion of small amounts of tantalum in the resulting nickel-aluminum coating composition as a modifier. Surprisingly, because of the sintering temperatures used and the rapidity with which aluminum diffuses into the nickel-base substrate at such temperatures, it is necessary that the powder mixture forming the coating composition contain from 50 to 80% by weight of tantalum in order to achieve amounts of tantalum modifier in the final coating that will be sufficient to be useful.

It has been found that best results are achieved when a powder composition of tantalum and aluminum powders corresponding to stoichiometric TaAl is used, or a powder mixture by weight of about 70% tantalum and 30% aluminum. The resultant coating formed after sintering consists essentially of a tantalum-modified nickel-aluminum composition. Even with the considerable preponderance of tantalum by weight in the original coating composition, the resultant sintered coating contains only from 1 to 10% tantalum by weight of the total coating composition including aluminum, nickel in intermetallic compounds with aluminum, and tantalum itself. The usual amount of tantalum in the resultant coatings is about 2% by weight, although it is desirable to get as much tantalum modifier as possible into the nickelaluminum coating composition.

The tantalum modifier has the unexpected beneficial result of raising the melting point of the nickel-aluminum composition to a striking degree, and conjointly increasing the diffusional stability of the coating thereby preventing diffusion of aluminum into the nickel-base substrate. An aluminum rich zone results at the surface of the coating, and this promotes formation of a nickel-aluminum spine] when the substrate is exposed to an oxidizing environment at an oxidizing temperature.

Although a high purity hydrogen atmosphere furnace is preferred for forming the coating compositions of this invention, it is also possible to use a high purity argon atmosphere or a vacuum atmosphere.

It is important that tantalum and aluminum powders used in forming the coatings of this invention be of the highest purity obtainable. Tantalum and aluminum powders should be 99% or more pure. Inclusion of even small amounts of silicon is undesirable. Even though gross oxidation resistance may not be affected, silicon causes a severe and unacceptable reduction in the melting point of the coating through introduction of low melting phases found between aluminum and silicon as well as between nickel and silicon. Silicon also adversely affects the ductility of the coating.

Titanium should also be avoided, since it confers no benefit to the coating and may lower its heat resistance. It may also degrade the beneficial diffusion arresting effects that tantalum has on aluminum. If metal powders of the highest purity attainable consistent with economic factors are used, the danger of undesirable side effects from additional elements introduced as impurities is greatly reduced.

For a clearer understanding of the invention, specific examples of the invention are set forth below. These examples are merely illustrative and are not to be understood as limiting the scope and underlying principles of the invention in any way.

EXAMPLE I A mixture of high purity metallic powders of the following composition was prepared:

A liquid dispersant containing the following proportions of ingredients was prepared:

Acetone milliliters 50 Amyl acetate do 325 Nitrocellulose grams 7 /2 A ball mill container was then filled with a minimum of 5 pounds of 1 inch diameter china milling balls or enough balls to fill the container /3 full. A measured quantity of tantalum and aluminum powder mixture was then placed in the container and a measured amount of liquid dispersant was added until the balls, powder and liquid in the container filled it from /2 to /3 full. The contents of the ball mill were then milled to a slurry for from 12 to 24 hours at about 14 r.p.m. per jar.

For good results the viscosity of the slurry was kept at 700:200 centipoises at F. as measured with a Brookfield Viscometer using the No. 1 spindle at 20 r.p.m. or equivalent. If necessary, the viscosity was reduced by adding additional acetone and mixing thoroughly once more either by ball milling or by rotating the container without milling media for approximately 1 hour. If the viscosity was too low, it was increased by adding additional tantalum and aluminum powder mixture and milling as described or by blending with a slurry of a higher viscosity and milling as described for about 1 hour.

The resulting dispersant was then sprayed onto an erosion bar made of a nickel-base alloy called SM200 and having the following composition by weight:

The solvent was then evaporated by allowing the specimens to stand at room temperature.

Following evaporation of the solvent, the erosion bar with its adhered applied powders was placed in a hydrogen atmosphere cyclic furnace. High purity hydrogen having a dew point of about 60 F. was introduced into the furnace to a pressure slightly exceeding atmospheric. The temperature of the furnace was then raised to 1975 F.

Sintering was carried out for 4 hours at 1975" F., and under a pressure of hydrogen slightly greater than atmospheric. After sintering the hydrogen atmosphere was maintained and the erosion bar was cooled to 500 F. It was then removed from the furnace and allowed to cool to room temperature.

The resultant coating was about 2 mils thick and possessed room temperature ductility. As stated earlier, the coatings of this invention should be from 1 to 3 mils thick, and preferably about 2 mils thick, because in layers of this thickness they adhere strongly to and take on the characteristics of the substrate, maintaining room temperature ductility and avoiding brittleness.

The coating of this example was subjected to repeated thermal shock testing by subjecting it to a temperature of 2100 F. for 1 /2 minutes and and then to a pressurized cold air stream for /2 minute. Once in every 100 cycles the specimen was allowed to remain at room temperature for 30 minutes. After 800 cycles the specimen was still unfaile-d and thus displayed excellent resistance to thermal shock.

Erosion bar specimens prepared as in Example I were rotated at 1750 r.p.m. in a gas stream comprising the combustion products of JP-S (a high flash point kerosenetype jet fuel) and air providing an atmosphere closely approximating that encountered in a gas turbine engine. Such oxidation-erosion testing was conducted on these erosion bar specimens at 2000 F. for 100 hours and followed by another 100 hours at 2100 F.

To graphically illustrate the superiority of the coatings of the present invention over well known existing coatings, erosion bar specimens made up according to Example I were simultaneously oxidation-erosion tested with SM200 nickel-base alloy erosion bars coated and heat treated as set forth in Table I below: a

Table I Coating: Heat treatment Example I 1800 F., 4 hours (argon). MoSi (prealloyed) 1800 F., 4 hours (argon). NiAl (prealloyed) 2250 F., 4 hours (vacuum). Ni-20Cr (by weight) 2250 F., 4 hours (vacuum). TaCr 2200 F., 4 hours (argon).

Weight change results of such testing are shown in FIGS. 1 and 2. FIG. 1 shows weight gain or loss for first 100 hours at 2000 F., and FIG. 2 shows weight change during second 100 hours at 2100" F. The coatings of this invention are represented by the specimen designated Example I in the drawings.

Visual examination of erosion bars after testing (see FIG. 6) showed general erosion of all coated specimens except for the bars coated in accordance with the teachings of this invention and designated Example I. The Example I bars were still in excellent condition.

FIG. illustrates the superior protection aiforded by the coatings of this invention over 1) an uncoated SM200 substrate and (2) SM200 coated with Al-IOSi (by weight) coating after oxidation-erosion testing at 2000 F. for 100 hours.

FIG. 3 is a photomicrograph of a transverse section taken from a cool section of the erosion bar leading edge and enlarged 500 times to show the composition of the coating after oxidation-erosion testing. This photomicrograph (FIG. 3) shows a pellicular film of oxide on the outer surface of the coating and underneath this a primary coating zone consisting essentially of a tantalummodified nickel-aluminum composition similar to NiAl dispersed in a tantalum-modified nickel-aluminum composition matrix. Beneath the primary coating zone is a diffusion zone consisting essentially of subaluminides of the substrate and including compounds intermediate in composition between the substrate and the primary coating zone. It will be noted that in this cool area of the erosion bar the primary coating zone exhibits an essentially fine grain structure and almost uniform dispersion of small tantalum-modified grains of NiAl throughout the nickel-aluminum composition matrix.

The photomicrograph of FIG. 4 is a transverse section taken from a hot section of the erosion bar leading edge and enlarged 500 times. It will be noted that this photomicrograph shows a dilferent structure than that shown in FIG. 3. FIG. 4 thus displays a tantalum-modified coating structure having a pellicular film of oxide at the surface, and a primary coating zone characterized by large blocks of a nickel-aluminum composition similar to NiAl forming an intermediate coating zone, an aluminum rich outer coating zone of a nickel-aluminum composition similar to Ni Al and a nickel rich inner coating zone similar to Ni Al. In other respects the coating structure of FIG. 4 resembles that of FIG. 3.

The differences between FIGS. 3 and 4 illustrate a phenomenon of the present invention by which the coating structure changes as a function of time and temperature. If the coating shown in FIG. 4 were exposed to an additional hours of oxidation-erosion at about 2100 F. the large blocks of NiAl type composition forming the intcrmediate coating zone would coalesce to form an essentially continuous stratum of NiAl between the two strata forming the outer and the inner coating zones. This structure is the most desirable for the coatings of this invention, because it exhibits greatest ductility without sacrificing any protective qualities of the coating.

In accordance with the invention, it is thus possible to create this latter form of coating by treating the as-sintered coating of Example I at a temperature of from 2150 to 2250 F. for a time of about 250 hours. At lower temperatures the same changes in coating structure take place but longer periods of exposure are required to effect them. For example, a change in coating structure effected in 100 hours at 2200 F. might require 15,000 hours of exposure at 1800" F.

At 2200 F. the following changes in coating structure have been observed:

(1) In the as-sintered condition the coating has essentially no definite structure other than a fine grain dispersion of tantalum-modified NiAl in a nickel-aluminum composition matrix.

(2) After 25 hours, a larger grain structure tantalummodified NiAl begins to appear.

(3) After 100 hours, the enlarged grain structure changes to a structure in which large blocks of tantalummodified NiAl are present.

(4) Finally, after about 250 hours of exposure, the coating achieves a layered structure having three distinct strata with tantalum-modified NiAl as the intermediate stratum or layer.

The structure of the coating is continuously changing at 2200 F. because of accelerated diffusion that takes place at this temperature. At lower temperatures essentially the same changes take place but at much slower rates.

Visual examination of various erosion bars after oxidation-erosion testing showed that the prior art coatings had sulfered markedly from general erosion except the coatings of this invention, designated Example I in the drawings. The Example I coatings remained in good condition 1 1 even after 100 hours at 2000 F. followed by 100 hours at 2100 F. (see FIGS. 5 and 6).

Although the exact mechanism of modification or change wrought by tantalum on the nickel-aluminum composition coating of this invention is not well understood, it is believed that tantalum atoms are substituted for nickel atoms to a small but important extent in the lattice structure of the nickel-aluminum intermetallic compounds. All references to tantalum content in the specification and claims are based on weight percent of tantalum to the total composition including weights of nickel, aluminum and tantalum present in the coating structure outward from the difiusion zone.

EXAMPLE II A metal powder mixture of the same composition as Example I was sintered onto a nickel-base alloy known as Inco 713 having the following composition:

Ni-.20C-l4Cr-4.5M0-1.0Ti-6Al-2Cb+Ta-.l2Zr-O15B Results similar to those of Example I were obtained. EXAMPLE III A metal powder mixture having the same composition as Example I was sintered onto a nickel-base alloy known as IN 100 having the following composition:

A metallic powder mixture of the same composition as Example I was sintered onto a nickel-base alloy known as Waspaloy having the following composition:

Results similar to those of Example I were obtained.

The invention in its broader aspects is not limited to the specific details shown and described but departures may be made from such details within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is:

1. As an article of manufacture, a high-temperature oxidation-resistant metal composite comprising: (a) a nickel-base alloy substrate having nickel as its principal component; and (b) a coating bonded to the substrate and consisting essentially of nickel, aluminum and from 1 to 10% by weight of the total coating composition of tantalum, said coating having an atomic ratio of nickel to aluminum of from 2:3 to 3:1.

2. The article of claim 1 which has a diffusion zone beneath the inner zone, said diffusion zone consisting essentially of subaluminides of the substrate.

3. The invention as defined in claim 1, in which the tantalum content is about 2% by weight of the total shell composition.

4. The article of claim 1 in which said coating is stratified and comprises an aluminum-rich nickel-aluminum outer zone, and a nickel-rich nickel-aluminum inner zone adjacent said substrate; the coating composition in each of said zones consisting essentially of nickel, aluminum and from 1 to 10% by weight of tantalum.

5. The article of claim 4 in which the atomic ratio of nickel to aluminum in said outer zone is 2:3, and the atomic ratio of nickel to aluminum in said inner zone is 3:1; said article also having an intermedate coating zone between said inner and outer zones, consisting essentially of nickel, aluminum and from 1 to 10% by weight of tantalum, and having an atomic ratio of nickel to aluminum of 1:1.

6. As an article of manufacture, a high-temperature, oxidation-resistant metal composite, comprising: (a) a nickel-base alloy substrate having nickel as its principal component; and (b) a coating bonded to the substrate and consisting essentially of a nickel, aluminum and from 1 to 10% by weight of the total coating composition of tantalum, said coating having an atomic ratio of nickel to aluminum of from 2:3 to 3:1, and having an outer surface film bonded to it consisting essentially of a nickelaluminum spinel.

7. The article of claim 6 in which the coating comprises a plurality of coating zones, including an outer coating zone having an atomic ratio of nickel to aluminum of 2:3, and intermediate coating zone having an atomic ratio of nickel to aluminum of 1:1, and an inner coating zone having an atomic ratio of nickel to aluminum of 3:1; each of said zones consisting essentially of nickel, aluminum and from 1 to 10% by weight of tantalum.

References Cited UNITED STATES PATENTS 2,542,962 2/ 1951 Kinsey -170 3,129,069 4/ 1964 Hanink 29-197 3,141,744 7/1964 Couch 29-197 HYLAND BIZOT, Primary Examiner. 

1. AS AN ARTICLE OF MANUFACTURE, A HIGH-TEMPERATURE OXIDATION-RESISTANT METAL COMPOSITE COMPRISING: (A) A NICKEL-BASE ALLOY SUBSTRATE HAVING NICKEL AS ITS PRINCIPAL COMPONENT; AND (B) A COATING BONDED TO THE SUBSTRATE AND CONSISTING ESSENTIALLY OF NICKEL, ALUMINUM AND FROM 1 TO 10% BY WEIGHT OF THE TOTAL COATING COMPOSITION OF TANTALUM, SAID COATING HAVING AN ATOMIC RATIO OF NICKEL TO ALUMINUM OF FROM 2:3 TO 3:1. 